1. Field of the Invention
The present invention relates to a method for producing an aromatic polycarbonate. More particularly, the present invention is concerned with a method for producing an aromatic polycarbonate, which comprises: reacting acetone with a phenol material to produce bisphenol A, and polymerizing the bisphenol A with diphenyl carbonate to produce an aromatic polycarbonate while producing phenol as a by-product, wherein the by-product phenol is recovered as a crude phenol product containing the by-product phenol as a main component and containing impurities, and the crude phenol product is used as at least a part of the phenol material for producing bisphenol A. According to the method of the present invention, a crude phenol product as such, containing, as a main component, by-product phenol (occurring during the polymerization reaction for producing an aromatic polycarbonate) and containing impurities, can be utilized for producing bisphenol A, finally for producing an aromatic polycarbonate, without any purification or the like. Therefore, in the method of the present invention, not only can a necessity for complicated operations, such as a purification treatment, be reduced, but also the amount of wastes can be reduced and the yield of the aromatic polycarbonate, based on any of phenol and bisphenol A, can be improved.
2. Prior Art
In recent years, aromatic polycarbonates have been widely used in various fields as engineering plastics having excellent heat resistance, impact resistance and transparency.
With respect to methods for producing aromatic polycarbonates, various studies have heretofore been made. Of the methods studied, a process utilizing an interfacial polycondensation between an aromatic dihydroxy compound, such as bisphenol A {2,2-bis(4-hydroxyphenyl)propane} and phosgene, has been commercially practiced. However, the interfacial polycondensation process has problems in that it is necessary to use phosgene, which is poisonous, that a reaction apparatus is likely to be corroded with chlorine-containing compounds, such as hydrogen chloride and sodium chloride, which are by-produced, and methylene chloride which is used as a solvent in a large quantity, and that difficulties are encountered in separating and removing impurities, such as sodium chloride, and residual methylene chloride, which adversely affect properties of a produced polymer.
For solving the above-mentioned problems, various methods for producing an aromatic polycarbonate by a transesterification process by using a diaryl carbonate instead of phosgene have been proposed. For example, an aromatic polycarbonate can be produced by polymerizing bisphenol A with diphenyl carbonate (DPC) in molten state. In this method, a high polymerization degree of the polycarbonate cannot be achieved without distilling off an aromatic monohydroxy compound (such as phenol) produced as a by-product from the highly viscous molten polycarbonate. Therefore, this method has various disadvantages in the following points: (1) since a high temperature is needed for the polymerization, branching and/or crosslinking are likely to occur as side reactions, so that it is difficult to obtain a high quality polycarbonate; (2) undesirable discoloration of the polymer cannot be avoided; and the like {see "Purasuchikku Zairyo Koza (5), Porikaboneto Jushi (Lecture of Plastic Materials (5), Polycarbonate resins)", pages 62-67, written by Mikio Matsukane et al. and published by Nikkan Kogyo Shinbunsha (1969)}. For solving the above-mentioned problems, various attempts have been made with respect to catalysts, stabilizers, polymerization methods and the like. Especially, in the specification of WO 95/03351 (corresponding to U.S. Pat. No. 5,596,067), the present inventors have disclosed a method for producing an aromatic polycarbonate by conducting a polymerization reaction in which a molten mixture of an aromatic dihydroxy compound and a diaryl carbonate or a prepolymer obtained by the reaction of an aromatic dihydroxy compound with a diaryl carbonate is allowed to pass downwardly through a perforated plate and fall freely, so that polymerization is effected during the free-fall. By this method, a high quality aromatic polycarbonate with no discoloration can be obtained. Further, U.S. Pat. No. 5,589,564 discloses a method for producing an aromatic polycarbonate in which the above-mentioned molten mixture or prepolymer is allowed to fall along and in contact with a guide (wire) (i.e., wire-wetting fall), so that polymerization is effected during the wire-wetting fall. By this method, a high quality aromatic polycarbonate with no discoloration can be obtained. Also, a method has been proposed for producing an aromatic polycarbonate by solid phase polymerization of a diaryl carbonate with an aromatic dihydroxy compound (e.g. U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,214,073), and a high quality aromatic polycarbonate can be obtained by the method.
Thus, it has become possible to obtain a high quality aromatic polycarbonate by the transesterification process. However, differing from the case of the phosgene process, in producing an aromatic polycarbonate by the transesterification process, phenol is necessarily by-produced. From the economical view-point, it is desired to utilize the by-produced phenol. Therefore, for example, in Unexamined Japanese Patent Application Laid-Open Specification Nos. 9-255772 (corresponding to U.S. Pat. No. 5,747,609) and 1060106, the by-produced phenol is recovered and utilized in the production of diphenyl carbonate (DPC). However, as described in these documents, before the by-produced phenol is utilized in the DPC production, it is necessary to subject a crude phenol product (containing the by-produced phenol) recovered from the aromatic polycarbonate production stage to purification by, for example, distillation, so as to prevent a clogging of the conduits in the DPC production stage and to improve the quality of DPC and the quality of the final aromatic polycarbonate. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 10-60106 discloses that the impurities contained in the crude phenol product are as follows: a dihydroxy compound: 50 ppm or less; a transesterification catalyst (i.e., a nitrogen-containing basic compound or at least one compound selected from the group consisting of an alkali metal compound and an alkaline earth metal compound): 1 ppm or less; a mixture of phenyl salicylate, o-phenoxybenzoic acid and phenyl o-phenoxybenzoate: 50 ppm or less; and a polycarbonate oligomer: 100 ppm or less. This means not only that in the DPC production stage or in a stage before the DPC production stage, it is necessary to effect a purification of a crude phenol product by using, e.g., a phenol distillation column or a phenol rectification column, but also that complicated operations are necessarily added.
Further, a liquid withdrawn from the bottom of the above-mentioned phenol distillation column or phenol rectification column contains high concentrations of bisphenol A, diphenyl carbonate and an aromatic polycarbonate oligomer and the like separated from the crude phenol product (containing by-produced phenol). As a result, conventionally, the yield of the aromatic polycarbonate, based on either phenol or bisphenol A becomes low, and the necessity for disposal of the liquid withdrawn from the bottom of the phenol distillation column or phenol rectification column has been a problem.
As apparent from the above, a technique which makes it possible to efficiently utilize the by-produced phenol necessarily occurring during the polymerization reaction for the production of an aromatic polycarbonate by the transesterification process has not yet been established.